I was sitting in the tub for way too long, and eventually got out, reasoning to myself that it would be frigid if my mind had been allowed to drift any longer. What makes water (or any liquid) normalize to its surroundings? I remember doing chemistry problems where I had to find the resultant temperature if 5mL of boiling water was put into 2mL of ice water, but that’s as far as I got scientifically.
This is probably really simple, but what’s going on?
The heat is transferred from the water to the air in the room and thence to the outside air by four processes: evaporation, conduction, convection and radiation.
Evaporation: The molecules in the water are zipping around at a variety of speeds, some of them way, way up in the many kilometers/hour. If that velocity happens to be toward the surface, and for some molecules it will be, and is high enough the molecule will escape the liquid and become a molecule of water vapor in the air of the bathroom. Since it is the higher speed, i.e. hotter, molecules that escape as vapor the molecules left in the water are the cooler ones and the water temperature goes down.
Conduction: The molecules of the water are bouncing against the molecules of the side of the tub and since the water molecules are on average hotter than the molecules of the tub side the tub side gets warm. Since the tub molecules on the water side of the tub are now on average hotter than those in the tub side further away, the molecules further away get warm and the heat progresses through the material of the tub always going from high temperature to low as Garfield225 said.
Eventually the side of the tub in contact with the bathroom air gets warmer than that air and so the tub molecules warm up the air molecules. Here is were convection enters the picture.
Convection: Back in the water. The water molecules that have given up their heat to the tup material are now colder than the average of the water and so the water next to the tub side is denser and sinks to be replaced by new and warm water which in turn bump into the tub and heat it up, and so on and so on.
On the other side, the air next to the tub is now warmer than the average of the room air and so is less dense. It therefore rises and is replace by cooler air which is warmed by contact with the tub and so on and so on.
Radiation: All this while the water in the tub is warmer than its surroundings and so a certain amount of the heat in it radiates away as infre red electromagnetic waves.
These processes all go on until the water is as the same temperature as its surroundings.
But as far as why it does all these things:
Heat is just vibration of the atoms and molecules, in a random jiggling. If you have the shakes, and you touch something that is moving less than you are, the tendency will always be to make it move more than it was, and to reduce your own shaking. Maybe by a little, maybe by a lot, but the thing that moves more will always tend to impart motion to the thing that moves less, and not the other way around.
We talk about heat as if it is some kind of stuff, like you could bottle Essence of Heat or something. But it’s just a large scale statistical phenomenon that, at the smaller scale, is only ordinary movement of atoms and molecules. One can say many amazing things about this process, but fundamentally that is all it is.
Technically, we’re talking about the
Zeroth Law of Thermodynamics , which was added after all the cool numbers were taken.
While everyone has done a bang-up job of explaining how it happens, the answer to “why,” ultimately comes down to the fact that, basically, it has to. Heat energy always “flows” from high to low, and not vice-versa. Why? Because. Just because. The zeroth law is axiomatic; derived from the observation that it always does this. It can’t be derived from anything simpler.
Read “Energy, Engines, and Entropy,” by John Fenn for a pretty straightforward, yet readable treatment of thermodynamics.
Isn’t this making it more mysterious than it has to be? Isn’t it simply a case of heat being a term for something and cold being a term for an absence of something. Speaking of cold flowing towards hot is therefore a nonsense, because cold doesn’t exist to flow.
Well, yes. But it’s not a universal true that high concentrations of “something” flow away and are diluted by contact with low concentrations of that same “something.” This often doesn’t happen to gas in interstellar space, for instance: rather than expanding out into the surrounding vacuum, high concentrations of gas become more concentrated due to gravity. It’s obvious from observation that heat isn’t a “something” that works this way, but this observation does yield non-trivial information.
Sorry for the histrionics, but something as commonplace as a bathtub full of hot water getting cold is, when you take a closer look, extremely mysterious, or was to early scientists, who started by asking precisely the same question as the OP. To find an answer, they had to create a new branch of physics to explain it.
Thanks for the compliment, but I think the fact that my tub water is jiggling on a molecular level solves that mystery.
>Speaking of cold flowing towards hot is therefore a nonsense, because cold doesn’t exist to flow.
I disagree. Speaking of heat flowing towards cold or cold flowing towards hot are both applying a statistical appraisal of motion, or a human sensation, or some other abstract construct, to describe an observation. “Heat” is hardly a more real description than “cold”. I think the only meaningful distinction between them is that there is a meaningful zero for “heat” that usually doesn’t require the use of negative numbers, which isn’t true for cold. And even so there are some situations that require negative temperatures even when using the idea of heat.
Don’t get me wrong - I believe in heat - I just think picturing it as the underlying truth, rather than a way of managing what we observe, is putting too much faith in our mental picture.
I agree with this about heat. If you compute the input energy and the useable output energy for a sytem it turns out that the input is always at least a little bigger than the useable output. That unuseable part is heat and it raises the temperature of the system.
In one really crude sense heat is a bookkeeping device made necessary by the first law of Thermodynamics.
Heat was first thought of as some sort of fluid within objects, when it was thought about at all. But then some guy, whose name escapes me at the moment, was boring cannons using a teams of horses to turn the drilling machine. He had to continually keep pouring water on the work-piece and the drill to avoid burning them up. It seemed odd that an objects like a cannon and a drill bit could contain what seemed to be an endless supply of this fluid, heat. He thought about it and came to the conclusion that the heat must be coming from the work done by the horses and that heat must be a form of energy. When steam engines came along the flow of heat became a hot topic and then the atomic theory finally allowed scientists to get to the bottom of what this thing heat really is.
“Cut and paste” is a curse and “Preview Post” is a friend, but only if you pay attention when previewing. On preview you have to pretend you are trying to correct writing done by someone you don’t like and you hope to find things you can ridicule.
Dele the word “an.”
Represent cold flowing towards heat in a physical fashion. What is going on at a molecular level?
>Represent cold flowing towards heat in a physical fashion. What is going on at a molecular level?
The stability of the quieter molecular structures is spreading to, and stabilizing, the noisier structures.
I think this is no less contrived than the representation of heat flowing into cold as being the vibration of the noisier structures spreading into the quiet of the more stable structures.
That would be Sir Benjamin Thompson, Count Rumford.
Fascinating fellow. Born in Massachusettsin 1753, worked for the British during the Revolutionary War, later lived in England (where he was knighted) and then Bavaria (where he was made a Count of the Holy Roman Empire). He invented many diverse things, including a soup and a fireplace (both of which are named after him, and there aren’t many folk of which you could say that!). Co-founded the Royal Institution, endowed the Rumford Medal for the Royal Society, founded a professorship at Harvard, and ended up marrying Lavoisier’s widow and living in Paris until his death in 1814.
And baking powder. Don’t forget the baking powder!!
Although Rumford Baking Powder was named after the Count, it wasn’t invented by him but by Eben Horsford, who had occupied the Rumford chair at Harvard and named his invention after his benefactor.
Rumford did invent a drip coffee-pot, however.
Here is a PDF biography, “Count Rumford: Scientist, Humanitarian, Administrator, Spy, and Soldier of Fortune”, that goes into more details of his life, inventions, and political intrigues.
I found it, thanks to Understanding Physics by Isaac Asimov (Barnes & Noble), a fine book that I recommend to all non-scientists who would like to know more about physics.
[From the very start of the 19th century evidence that heat was not a fluid began to mount]
“In 1798 …Benjamin Thompson, Count Rumford (1753-1814), a Tory exile from the United States, was boring cannon in the service of the Elector of Bavaria. He noted that great quantities of heat were formed. Neither the cannon being bored nor the boring instrument used was at more than room temperature to begin with, and yet the heat developed by the act of boring was sufficient to bring water to a boil after a time; and the longer the boring was continued the more water could be boiled. It almost appeared as though the quantity of heat contained within the cannon and borer was infinite.”
“If heat were a fluid, and a form of matter, then to suppose it were formed in the act of boring raised a difficulty. Already, the French chemist Antoine Laurent Lavoisier (l743-1794) had established the law of conservation of matter, according to which matter could be neither created nor destroyed; and there was an increasing tendency among scientists to believe this generalization to be valid. If heat were being formed, then it must be something other than matter. To Rumford, the most straightforward possibility was that the motion of the boring instrument against the metal of the cannon was transformed into the motion of small parts of both borer and metal, and that it was this internal motion that was heat.”
“This notion was largely disregarded during the following decades. The assumption that small parts of an object might be moving invisibly seemed in 1800 to be just as difficult to accept as the assumption that matter was being created, perhaps even more difficult. A decade after Rumford’s experimenting, however, the atomic theory was advanced and began to increase in popularity. By the internal movements of matter, one now meant the motions or vibrations of the atoms and molecules making it up, and the assumption of such motion became continually more acceptible. In the 1840’s [James Prescott] Joule’s (1818-1889) experiments in converting work to heat extended Rumford’s observations and made the victory of the atomic motion view of heat inevitable. Finally, in the 1860’s, the kinetic theory of gases and the concept of heat as a form of motion on the atomic scale were established rigorously by [James Clerk] Maxwell (1831-1879) and [Ludwig] Boltzman (1844-1906)”
Whats does stability mean in this context?